1. Field of the Invention
The present invention relates to a transistor-transistor logic (TTL) circuit. More particularly, it relates to a TTL inverter having a transient state of a high impedance output for reducing the transient current.
Transistor-transistor logic inverters are generally used as basic elements in semiconductor integrated circuits. Inverters constructed by PNP transistors or NPN transistors are known. In the following description, TTL inverters constructed with NPN transistors are explained. However, the present invention is also applicable to TTL inverters constructed with PNP transistors. 2. Description of the Prior Art
A conventional TTL inverter comprises a single phase-splitter transistor for splitting the phase of an input signal, which is applied to the base of the phase splitter transistor, into a non-inverted phase at its emitter and an inverted phase at its collector; an inverted signal output transistor for inverting the signal of the noninverted phase; and an off buffer circuit connected in series with the inverting transistor for receiving the inverted phase signal from the collector of the phase splitter transistor, whereby the input signal applied to the input end is inverted at the output end. In principle, the off buffer circuit and the inverted signal output transistor are alternately turned on and off in response to the outputs of the phase splitter transistor.
In the above-mentioned conventional TTL inverter, however, there are the following disadvantages. That is, in a transient period during which the output signal changes from an "H" (high) level to an "L" (low) level or vice versa, there is a period when both the off buffer circuit and the inverted signal output transistor are simultaneously conductive so that a large transient current flows therethrough. This transient current brings about not only an increase in power consumption but also a fluctuation of the power supply voltage, the fluctuation causing error operations of the external or internal circuits. In a large-scale integrated logic circuit, the transient currents in the inverters accumulate, resulting in a more serious problem in the above-mentioned error operations.
When the off buffer circuit comprises a single transistor for obtaining an "H" level of the output voltage higher than the "H" level given by the off buffer circuit comprises two transistors in a Darlington connection, a conventional TTL inverter requires a noninverting gate connected between the collector of the phase splitter transistor and the off buffer circuit. The noninverting gate comprises a number of elements. Because of the number of elements, the conventional TTL inverter of this type has a low integration degree and a low operating speed.